Step 5: The Microphone

Step 6: Using It

To use it, just plug the transmitter into any audio source (CD player, radio, etc.), and turn it on! It will modulate the sound signals into light, an...

Step 4: The Receiver

The receiver is so simple, a monkey could do it. All you have to do is solder the CdS photocell in series with the battery pack (3 volts), and then solder the two resulting leads to another mono jack. Use the biggest photocell you can find. Also note that it does not matter which way you connect the photocell; it has not plus (anode) or minus (cathode) sides.

Here's something for a transmitter you might like to experiment with. Glue a little mirror on a speaker and shine a light source onto it such that it reflects to the photocell receiver. Make sure just the edge of the light hits the photocell (about half way) so that, as sound comes from the speaker, the light covers the photocell a varying amount. The idea is the reflected light from the speaker will cause the edge of the light beam to just cover about half the photocell with no sound and vary in the area of cell covered as sound causes the beam to move. For this, a larger area photocell might be good. Perhaps a photovoltaic cell as opposed to a photoresistive cell. Any source of light could do, even sunlight. A laser would be good as it remains in a nice tight bundle over a long distance, making it easier to "calibrate." One consequence of this setup is that it will become more sensitive the greater distance between the transmitter and receiver. That is, it will take less movement of the speaker to cause a given change on the photocell the farther away it is from the transmitter. Also, the intensity of the light source is independent of the power required to modulate it. FYI, this sort of thing makes a good vibration sensor, such as for a seismic acivity detector. The greater separation, the more sensitive it is.